In the related art, it is known that a phenomenon called a current collapse occurs in which on-resistance changes when a voltage is applied to a field effect transistor using two-dimensional electron gas (2 DEG) generated on a hetero interface of a nitride semiconductor multilayer structure formed of a plurality of layers having different compositions; however, the current collapse may be suppressed by laminating a silicon nitride (SiN) film on the surface of the nitride semiconductor. In this regard, an effect of suppressing the current collapse is affected by the properties of the SiN film, and thus various research and development have been conducted.
The SiN film with a high effect of suppressing the current collapse has a defect in that current leaks. Accordingly, an idea has been proposed that the SiN film laminated on the surface of the nitride semiconductor has a two-layer structure including a layer which contains a large amount of hydrogen and has a function of suppressing the current collapse, and a layer which contains less hydrogen and has a function of suppressing the leakage current.
Examples of the nitride semiconductor device in which the SiN film having such a two-layer structure is laminated on the surface of the nitride semiconductor include a semiconductor device which is disclosed in Japanese Unexamined Patent Application Publication No. 2008-205392 (PTL 1). In the semiconductor device disclosed in PTL 1, a non-stoichiometry first SiN film having a high concentration of Si—H or N—H and a substantially stoichiometry second SiN film which has a small quantity of Si—H or N—H bonds and is excellent in the insulation properties are laminated on a surface of a compound semiconductor region including a GaN electron transit layer, an AlGaN electron supply layer, and a GaN surface layer, thereby intending to suppress both of the current collapse and the leakage current.
In addition, examples of the aforementioned nitride semiconductor device also include a semiconductor device which is disclosed in Japanese Unexamined Patent Application Publication No. 2009-164300 (PTL 2). In the semiconductor device disclosed in PTL 2, a stoichiometry first SiN film which has a low total concentration of the added concentration of Si—H bonds and concentration of N—H bonds and is excellent in the insulation properties, and a non-stoichiometry second SiN film which has a high total concentration of the added concentration of Si—H bonds and concentration of N—H bonds and is excellent in stabilizing a surface of the compound semiconductor are laminated on the surface of a compound semiconductor multilayer structure including a GaN electron transit layer, an AlGaN electron supply layer, and a GaN surface layer, thereby intending to suppress both of the current collapse and the leakage current.
However, the semiconductor devices in the related art disclosed in PTL 1 and PTL 2 have the following problem.
That is, it is confirmed that even if two layers of the SiN films are laminated on the surface of the compound semiconductor region including the above-described nitride semiconductor, when a high temperature and high voltage stress test is conducted, the semiconductor device has the leakage current increased and is not able to be used as a switching device for a long period time.